Purinoreceptors bind to and are activated by a variety of both ribosylated (nucleotide) and non-ribosylated (nucleoside) purines. This distinction has been used to classify these receptors into two broad groups: the P1 receptors (A1, A2a, A2b, and A3), which bind to and are activated by the nucleoside adenosine, and the P2 receptors, which comprise a second, more diverse class of receptors which are activated by a wide variety of nucleotides including ATP, ADP, UTP, and UDP. The P2 receptors can be further subdivided into two distinct types of receptors; the ionotropic P2X receptors that mediate cation flux across cellular membranes in response to ATP and the metabotropic P2Y family of receptors which are G-protein coupled receptors. In humans, the P2Y family of receptors is generally considered to consist of seven distantly related members; P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, and P2Y13 (Boeynaems, J. M. et al. Drug Development Research 2001, 52, 187-9). In addition, an eighth receptor, P2Y14, has been considered by some to be a member of this class although it does not respond to ribosylated nucleotides and is activated by UDP-glucose (Abbracchio, M. P. et al. Trends Pharmacol. Sci. 2003, 24, 52-5).
Several studies have suggested that modulators of specific members of the P2Y family of receptors could have therapeutic potential for the treatment of a variety of disorders (for review see Burnstock, G. and Williams, M. J. Pharm. Exp Ther. 2000, 295, 862-9), including diabetes, cancer, cystic fibrosis, and the treatment of ischemia-reperfusion injury (Abbracchio M. P. and Burnstock G. Pharmacol. Ther. 1994, 64, 445-475). P2Y1 receptors, almost ubiquitous among human organs (Janssens, R. et al. Biochem. Biophys. Res. Comm. 1996, 221, 588-593) have been identified on microglia (Norenberg, W. et al. Br. J. Pharmacol. 1994, 111, 942-950) and on astrocytes (Salter M. W. and Hicks J. L. J. Neurosc. 1995, 15, 2961-2971). Extracellular ATP activates microglial and/or astrocytes via P2Y receptors and leads directly to the release of inflammatory mediators. Microglia and astrocytes are believed to play a role in the progression of Alzheimer's disease and other CNS inflammatory disorders such as stroke and multiple sclerosis.
Two members of the P2Y family, P2Y1 and P2Y12, are of particular interest as they have now both been shown to act as important receptors for ADP in platelets (Jin, J. et al. Proc. Natl. Acad. Sci. 1998, 95, 8070-4). ADP is a key activator of platelets and platelet activation is known to play a pivotal role in thrombus formation under conditions of high shear stress such as those found in the arterial circulation. In addition, more recent data has suggested that platelet activation may also play a role in mediating thrombus formation under lower shear stress such as that found in the venous circulation. ADP activates platelets by simultaneously interacting with both P2Y1 and P2Y12 to produce two separate intracellular signals which synergize together to produce complete platelet activation (Jin, J. et al. J. Biol. Chem. 1998, 273, 2030-4). The first signal arises from ADP driven activation of the P2Y1 receptor and can most easily be tracked by measuring the transitory increase in intracellular free Ca+2. This signal appears to mediate the initial shape change reaction and to initiate the process of platelet activation. The second signal appears to be derived from ADP activation of the P2Y12 receptor and serves to consolidate the process and produce an irreversible platelet aggregate. Using three structurally related but distinct inhibitors of P2Y1 (A3P5P, A3P5PS, and A2P5P), Daniel, J. L. et al. (J. Biol. Chem. 1998, 273, 2024-9), Savi, P. et al. (FEBS Letters 1998, 422, 291-5), and Hechler, B. et al. (Br. J. Haematol. 1998, 103, 858-66) were the first to publish the observation that the inhibition of P2Y1 activity alone could block ADP-driven aggregation independently of the P2Y12 receptor. Although inhibition of platelet reactivity is often thought of as firm evidence of an anti-thrombotic activity, these antagonists lacked the necessary pharmacological properties for in vivo study. The first direct demonstration that inhibition of P2Y1 activity could lead to an anti-thrombotic effect in vivo was reported by Leon, C. et al. Circulation 2001, 103, 718-23, in a model of thromboplastin induced thromboembolism using both a P2Y1 knock-out mouse and the P2Y1 antagonist MRS-2179 (Baurand, A. and Gachet, C. Cardiovascular Drug Reviews 2003, 21, 67-76). These results were subsequently extended to include the inhibition of both venous and arterial thrombosis in the rat (Lenain, N. et al. J. Thromb. Haemost. 2003, 1, 1144-9) and the confirmation of the phenotype of the P2Y1 knock-out mouse in a second laboratory using an independently derived animal (Fabre, J-E. et al. Nature Medicine 1999, 5, 1199-1202). These studies highlighted the need for more potent and selective P2Y1 antagonists and recently, using the P2Y1 antagonist MRS-2500 (Hechler, B. et al. J. Pharmacol Exp. Ther. 2006, 316, 556-563) succeeded in demonstrating strong antithrombotic activity for a selective P2Y1 antagonist in the mouse. Taken together, these data suggest that the discovery of novel P2Y1 antagonists with improved pharmaceutical characteristics could have significant utility in the treatment of a variety of thrombotic or thromboembolic disorders (see Gachet, C. et al. Blood Cell, Molecules and Disease 2006, 36, 223-227 for a recent review).